Incorporation of electrical elements such as heating elements and radio antennas, in vehicle glazings is a well established practice. Typically, such elements are connected to other electrical equipment on the vehicle, such as power supplies or amplifiers and radio signal receivers, by a soldered connection located on a surface of the vehicle glazing. The soldered connection might conveniently be located in the obscuration band—a region of dark ceramic ink in the periphery of the glazing. Thus the soldered connection is rendered invisible from the exterior of the vehicle, giving a more aesthetically pleasing result.
Traditional solders contain lead. These perform well but lead is known to be poisonous and lead free alternatives providing similar performance would be preferable. The use of lead free solders is known in the automotive industry and elsewhere but in many cases, they fall short of the performance demanded by vehicle manufacturers.
Often, lead free solders have a high in content. Such solders perform well in some respects, exhibiting good wettability and strength of bond, but for automotive applications, a high occurrence of stress faults are associated with such solders.
These stress faults may manifest themselves as blisters in the black ink of the obscuration band (particularly on toughened glass) or as cracks in a ply of laminated glazings. The stress faults are associated with differential thermal expansion and contraction between the solder and glass during the soldering operation.
Soldered joints may also be weakened by the temperature fluctuations experienced in service and by other mechanisms such as corrosion.
The resistance of lead free solders to thermal stress faults in particular may be increased by incorporating other elements such as indium in the solder alloy. However, there is an associated trade-off as the melting point of the solder, and its performance at higher temperatures, is reduced. The in-service temperatures encountered in some applications, e.g. in southern Europe, are frequently high enough to cause creep of the solder to the extent that the joint is unacceptably compromised.
Thus, a lead free solder which meets all of the requirements laid down by vehicle manufactures and legislators remains undiscovered.